Fiber-reinforced composite materials based on reinforcing fibers, such as carbon fiber and aramid fiber, are widely used in diverse fields to take advantage of their high specific strength and specific modulus. Examples include structural material applications, such as aircraft and motor vehicles, sports applications, such as tennis rackets, golf shafts and fishing rods, and general industrial applications. Typical methods to manufacture such fiber-reinforced composite materials include the prepreg-based method, which involves the lamination of prepregs, each sheet-like intermediate material obtained by impregnating reinforcing fibers with a matrix resin, and curing the laminate. The prepreg-based method is advantageous in that it facilitates the production of high performance fiber-reinforced composite materials by allowing strict control of the orientation of reinforcing fibers and offering a high degree of design freedom for laminate configuration. As prepreg matrix resins, thermosetting resins are mainly used from the viewpoint of heat resistance and productivity, and, of them, epoxy resins are particularly advantageously used from the viewpoint of bonding with reinforcing fibers and other mechanical characteristics.
In addition to ongoing moves to achieve weight reduction by replacing metals and other existing materials with fiber-reinforced composite materials, moves to pursue further weight reduction in fiber-reinforced composite materials themselves have been gathering momentum in recent years. Methods to achieve such weight reduction include the use of reinforcing fibers with higher modulus to reduce the weight of fiber-reinforced composite materials while maintaining their stiffness. However, increasing the modulus of reinforcing fibers tends to reduce fiber-direction compressive strength and other strength characteristics. To improve fiber-direction compressive strength and other strength characteristics, it is effective to improve the modulus of epoxy resins as matrix resins.
Techniques to improve the modulus of epoxy resins include the blending in of an inorganic filler, such as a carbon nanotube, or an amine-type epoxy resin with a high modulus.
For instance, patent document 1 shows that the use of an amine-type epoxy resin with a high modulus has improved the modulus of an epoxy resin composition, leading to a dramatic improvement in the fiber-direction flexural strength, a characteristic with a high degree of correlation with fiber-direction compression strength, of fiber-reinforced composite materials based on such an epoxy resin as their matrix resin. However, this method reduces the impact resistance of fiber-reinforced composite materials due to reduction in the toughness of the epoxy resin.
To improve the impact resistance of a fiber-reinforced composite material, it is necessary to improve the elongation of the reinforcing fibers and the toughness and plastic deformation capacity of the epoxy resin as ingredients of the fiber-reinforced composite material. Of these, improvement of the toughness of the epoxy resin is considered particularly important and effective.
To improve the toughness of an epoxy resin, methods such as the blending of a rubber component or thermoplastic resin with excellent toughness have so far been tried. However, since rubbers have much lower moduli of elasticity and glass transition temperatures than epoxy resins, blending a rubber in an epoxy resin has been observed to reduce the modulus and glass transition temperature of the epoxy resin, making it difficult to strike a balance between toughness and modulus. As thermosetting resin-based methods to dramatically improve the toughness of an epoxy resin, the blending in of a styrene-butadiene-methyl methacrylate copolymer and that of a butadiene-methyl methacrylate block copolymer have been proposed (patent documents 2 and 3). However, these methods are problematic in that they are associated with a reduction in processability due to reduced heat resistance or increased viscosity and a reduction in quality due to the formation of voids and the like. Moreover, the levels of modulus they have managed to produce are not quite satisfactory.
As a method to improve the balance between the modulus and toughness, the combining of a diglycidyl ether-type epoxy resin with a specific number average molecular weight, on the one hand, and an epoxy resin whose SP value differs from the epoxy resin over a certain range, on the other, has been disclosed (patent document 4). However, this method is also unsatisfactory as it is not only inadequate in terms of the balance between the modulus and toughness but also has a tendency to increase viscosity.